Usually, a device for displaying data in a portable instrument is formed by a liquid crystal cell. The display cell comprises a transparent front substrate having first transparent electrodes on an inner face, a transparent back substrate having at least one second transparent electrode on an inner face and a frame sealing the two substrates to define a closed space between the substrates. A substance occupies the closed space between the first and second electrodes. The optical or physical properties of the substance can be altered between the activated electrodes by a difference in the potential applied by control means. Via the action of the control means, the display cell, which also has polarizers, can display data, for example the time or date.
The electrodes of such liquid crystal cells must be continuously electrically powered so as to maintain or alter the display of data, which may constitute a drawback if the portable instrument is powered by a battery or accumulator of small size. Moreover, the liquid crystal cell does not allow the data display to be maintained when the battery is being changed.